Interlaminar Shear Strength and Failure Analysis of Aluminium-Carbon Laminates with a Glass Fiber Interlayer After Moisture Absorption

materials

Article Interlaminar Shear Strength and Failure Analysis of Aluminium-Carbon Laminates with a Glass Fiber Interlayer after Moisture Absorption

Jarosław Bienia´s*, Patryk Jakubczak , Magda Dro´zdzieland Barbara Surowska

Department of Materials Engineering, Faculty of Mechanical Engineering, Lublin University of Technology, Nadbystrzycka 36, 20-618 Lublin, Poland; [email protected] (P.J.); [email protected] (M.D.); [email protected] (B.S.) * Correspondence: [email protected]

Received: 11 May 2020; Accepted: 3 July 2020; Published: 6 July 2020

Abstract: This article presents selected aspects of an interlaminar shear strength and failure analysis of hybrid fiber metal laminates (FMLs) consisting of alternating layers of a 2024-T3 aluminium alloy and carbon fiber reinforced polymer. Particular attention is paid to the properties of the hybrid FMLs with an additional interlayer of glass composite at the metal-composite interface. The influence of hygrothermal conditioning, the interlaminar shear strength (short beam shear test), and the failure mode were investigated and discussed. It was found that fiber metal laminates can be classified as a material with significantly less adsorption than in the case of conventional composites. Introducing an additional layer of glass composite at the metal-composite interface and hygrothermal conditioning influence the decrease in the interlaminar shear strength. The major forms of damage to the laminates are delaminations in the layer of the carbon composite, at the metal-composite interface, and delaminations between the layers of glass and carbon composites.

Keywords: ﬁber metal laminates; moisture absorption; interlaminar shear strength; failure modes

1. Introduction Among the fiber metal laminates (FMLs), the most noteworthy are the laminates where the layers are made of carbon reinforced aluminium laminates (CARALL). CARALL are characterized by impact resistance, a high fatigue strength, and a low density with very low growth rates and efficient crack bridging [1–7]. An important aspect of FMLs is their high resistance to corrosion. This phenomenon is caused by the composite layers acting as a barrier in the corrosion process, which is limited to the outer metal layers [1,8–10]. Nevertheless, the CARALL may be more prone to corrosion. In this case, corrosion resistance is especially influenced by the electrical conductivity of carbon fibers. As a result, galvanic interactions occur at the metal-composite interface, especially in wet environments [11–13]. The absorption of moisture may lead to a decrease in the glass transition temperature; a plasticizing matrix; swelling, cracking, and delaminations at the fiber-matrix interface, modification of the local stress state; and rupture of the adhesive bonding in the system [7,14–16]. It was previously shown that moisture absorption in polymer composites decreases the mechanical properties and interlaminar shear strength [17–20]. In the literature, articles on the influence of environmental conditions on the properties of fiber-metal laminates can be viewed. Botelho et al. [7] investigated the influence of hydrothermal conditioning on the mechanical properties of CARALL by conducting tensile and compression tests. The authors suggest that the changes in the mechanical properties of CARALL are negligible, similar to the case of glass reinforced aluminium (GLARE) laminates [17]. In another

Materials 2020, 13, 2999; doi:10.3390/ma13132999 www.mdpi.com/journal/materials Materials 2020, 13, 2999 2 of 14 paper [21], the authors conducted research on hydrothermal effects by using the losipescu and interlaminar shear strength (ILSS) test for GALRE laminates. A decrease in shear strength was observed. However, it was stated that the influence of the hydrothermal effects on the shear strength results was inconsequential. The results of ILSS tests with a completely saturated GLARE specimen presented by Borgonje et al. [9] show that the presence of moisture has a significant influence on the obtained strength values. After exposure to a high-humidity environment, they observed a significant decrease in the tensile strength. In other studies, it was found that hygrothermal conditioning reduces the shear strength of CARALL laminates due to moisture absorption [5]. A decrease of the mechanical properties in CARALL after hygrothermal ageing was also observed by Pan et al. [10]. Research [22] on the effect of hydrothermal conditioning on the mechanical properties of fiber metal laminates such as GLARE and CARALL indicates a decrease in the tension resistance and ILSS of 15% and 9–11%, respectively, depending on the configuration. Ali et al. [15] investigated the seawater hydrothermal conditioning effects on the properties of titanium-carbon fiber/epoxy FMLs for marine applications. Their results showed that the percentage reduction in the mechanical properties of the Ti FML was lower in comparison to the conventional composites. The good properties of titanium fiber metal laminates after environmental conditioning in comparison to other FMLs were also confirmed by Silva et al. [23]. Poodts et al. [24] characterized a fiber metal laminate (with steel layers) for underwater applications. They observed that steel layers offer very good protection against water absorption and prevent the loss of the mechanical properties associated with it. Their results showed that saturated specimens exhibit a loss of the flexural modulus and shear strength of approximately 20%. It is very important to maintain proper bonding between the matrix and the steel layer. The authors found that water penetration at the steel-matrix interface induces delaminations, with subsequent degradation of the component. Accurate manufacturing processes ensuring a perfect bond between the steel layer and the matrix are thus strongly recommended. Singh and Angra [25] also studied the hygrothermal degradation of stainless steel fiber metal laminate conventional GF/E (glass fibre/epoxy) composites. In water, the compressive and tensile strengths of FMLs were reduced by 32.6% and 23.4%, respectively. The highest reduction in the compressive and tensile strengths of the GF/E composite was recorded as 36.8% and 29.8%, respectively, in water. Hygrothermally and unconditioned and conditioned tensile specimens fail due to delamination and fiber breakage, respectively, while compression specimens fail because of delamination between GF/E layers. Park et al. [26] investigated the effects of void contents on the long-term hydrothermal behaviors of GLARE laminates. They observed that the voids contribute to the water absorption properties and that the decrease in ILSS depends on the characteristic of water adsorption and voids. Large ILSS degradations of between 17% and 32% occurred for GLARE in comparison to the non-aged counterparts. Chandrasekar et al. [27] presented an experimental review on the mechanical properties and hydrothermal behavior of fiber metal laminates. Botelho et al. [28] also researched the hydrothermal effects on viscoelastic properties, such as the storage modulus (E0) and loss modulus (E”), for glass fiber/epoxy/aluminum laminate (GLARE). For GLARE laminates, the E0 modulus remains unchanged (49 GPa) during the cycle of hydrothermal conditioning. The research conducted by Zong et al. [29] showed a significant decrease in both the fatigue life and the tensile strength of Glare 4A laminates after being hygrothermally aged. Hu et al. [30] studied the long-term moisture absorption behavior of carbon-fiber reinforced polyimide (CF/P) and polyimide–titanium-based fiber metal laminates. The fiber metal laminates exhibited a slower moisture absorption rate in comparison to the pure composite. This was a result of the barrier function of the metal layers at both surfaces of the fiber metal laminates. The interlaminar and flexural strength of CF/P and fiber metal laminates decreased after hygrothermal ageing exposure. The mechanical properties of CF/P composites were reduced more than those of the fiber metal laminates. In the fiber metal laminates, debonding was only observed in the edge area. Molerio et al. [31] developed a new layer-wise mixed model for a coupled hygro-thermo-mechanical static analysis of fiber metal laminates, under a series of hygro-thermo-mechanical loadings. In another work, Molerio et al. [32] presented three-dimensional exact hygro-thermo-elastic solutions for multilayered plates: composite laminates, Materials 2020, 13, 2999 3 of 14

fiber metal laminates, and sandwich plates. These effects are demonstrated by the through-thickness distributions of displacements, stresses, temperatures, and weight percent moisture contents for the selected multilayered plates under different hygro-thermo-mechanical loadings, which may serve as 3D benchmark exact solutions. Works focused on improving the interlaminar strength can also be noted. For example, Garcia and co-authors [33] studied the effect of nylon nanofibers on the delamination resistance and dynamic behavior of GFRP composites. Due to the incorporation of nylon nanofibers in the nano-modified composites, a significant (27%) increase in the inter-laminar shear strength was obtained. This can be related to the presence of significant bridging phenomena due to the interaction between the nylon nanofibers and the epoxy resin. The authors concluded that improvement of the delamination resistance of GFRP laminates can be obtained by using nylon nano-fibers. In general, despite a significant resistance to environmental factors, moisture absorption may significantly influence the metal-composite interface in the process of degradation, as well as corrosion. It is also known that in fiber metal laminates, the interface between the aluminium and fiber reinforced polymers layer plays an important role in the stress transfer mechanisms of FML laminates [21]. Moreover, a significant damage process at the metal-composite interface, along with the occurrence of cracks and delaminations, is observed during the influence of moisture on fiber metal laminates. One of the methods used to increase the corrosion resistance in fiber metal laminates with carbon fibers is protecting the metal-composite interface as a result of isolating the composite layers from the aluminium layers. This is achieved by introducing thin thermoplastic layers (PEI, polyetheroimide) or GFRP between the aforementioned layers as electrical insulators [1]. Moreover, research on the application of additional hybrid sol-gel layers to the metal in order to increase its corrosion resistance and introducing an additional layer of epoxy resin at the metal-composite interface has been conducted [7]. Applying insulating interlayers, such as PEI or glass fiber layers, may cause a change in the mechanical properties of the FMLs. To the best of the authors’ knowledge, the behavior at the interface of a hybrid system Al-glass fiber-carbon fiber under the influence of environmental conditions has not been reported in the open literature. Therefore, there is a need to conduct research in order to improve our understanding of the environmental influence on the behavior of fiber metal laminates, which should be included in the process of designing composite structures. This paper presents results on the interlaminar shear strength of hybrid fiber metal laminates before and after environmental conditioning. The objective of this study was to conduct an analysis of the influence of an additional layer of glass fiber composite, applied between the aluminium and carbon fiber composite. Moreover, the characteristics of the types and mechanisms of damage as a result of the simultaneous influence of the moisture and an elevated temperature, including an analysis of the metal-composite interface, are presented.

2. Experimental Analysis

2.1. Material The subjects of the research were fiber metal laminates based on aluminum alloys and fiber reinforced polymers. The FMLs were manufactured by using 2024-T3 aluminium alloy (thickness of 0.3 mm) and unidirectional AS7J high-strength carbon and R-glass fiber reinforced epoxy resin prepregs (Hexcel, Stamford, CT 06901, USA). The nominal fiber content was about 60 vol.%. In order to ensure better adhesion of the components, the surfaces of aluminum sheets were anodized in chromic acid (CAA) and coated with EC 3924B corrosion inhibiting structural adhesive primer (3M, St. Paul, MN, USA). The anodizing process was carried out in accordance with the procedures used in the aviation industry. The process consisted of a number of stages, including rinsing, drying, alkaline degreasing, and sulffochromic etching. Anodizing was performed in CrO3 chromic acid anhydride at a temperature of 40 ◦C, a voltage of 20 V, and a time of about 45 min. After anodizing, another surface preparation step was carried out within 8 h. It consisted of applying a thin layer of epoxy-based, synthetic resin primer (EC 3924B, 3M, St. Paul, MN, USA) containing a corrosion inhibitor. Such a primer promotes long-term durability for bonded joints by Materials 2020, 13, 2999 4 of 14 Materials 2020, 13, x FOR PEER REVIEW 4 of 15 creatinginhibitor. an interface Such a p betweenrimer promotes the adhesive long-term and durability the metal. for The bonded primer joints curing by cyclecreating was an 30 interface min at 24 ◦C; then,between the process the adhesive was repeated and the at metal. 121 The5 Cprimer for 60 curing min. cycle was 30 min at 24 °C; then, the process ± ◦ wasFour repeated diﬀerent at 121 types ± 5 of°C laminatesfor 60 min. were manufactured: aluminium/CFRP (Al-C), aluminium/CFRP Four different types of laminates were manufactured: aluminium/CFRP (Al-C), with a GFRP interlayer (Al-GC), 2/1 FML with CFRP (FML-C), and 2/1 FML with a CFRP and GFRP aluminium/CFRP with a GFRP interlayer (Al-GC), 2/1 FML with CFRP (FML-C), and 2/1 FML with a interlayer (FML-GC). The conﬁgurations of the tested laminates and dimensions of the samples for CFRP and GFRP interlayer (FML-GC). The configurations of the tested laminates and dimensions of ILSSthe and samples hydrothermal for ILSS and conditioning hydrothermal are conditioning shown in Figure are shown1. in Figure 1.

FigureFigure 1.1. The configurations conﬁgurations of ofthe the tested tested laminates laminates..

TheThe unidirectional unidirectional laminate laminate configurationconfiguration used used is istypical typical of the ofthe ILSS ILSS test. test.In 2/1 In FML 2/1 laminates, FML laminates, moisturemoisture only only occurred occurred through through the the free free side side edges. edges. However,However, the one one-sided-sided configuration configuration allowed allowed the determinationthe determination of the behavior of the behavior at the metal-compositeat the metal-composite interface interface with with a significantly a significantly larger larger moisture absorptionmoisture surface absorption by the surface composite. by the This composite. can be This a representation can be a representation of metal-composite of metal-composite adhesive joints. Theadhesive laminates joints. were The produced laminates by were the autoclave produced method by the autoclave (Scholz Maschinenbau, method (Scholz Coesfeld, Maschinenbau, Germany). Coesfeld, Germany). The cure cycle was carried out at a heating rate of 2 °C/min up to 135 °C and The cure cycle was carried out at a heating rate of 2 C/min up to 135 C and held at this temperature held at this temperature for 2 h. The pressure and◦ the vacuum used◦ were 0.4 and 0.08 MPa, for 2respectively. h. The pressure and the vacuum used were 0.4 and 0.08 MPa, respectively.

2.2.2.2. Hygrothermal Hygrothermal Conditioning Conditioning In orderIn order to to assess assess thethe influence inﬂuence of ofthe the hygrothermal hygrothermal conditioning conditioning on the shear on the strength, shear the strength, fiber the ﬁbermetal metal laminate laminate specimens specimens were were exposed exposed to a to combination a combination of temperature of temperature and humidity and humidity in an in an environmentalenvironmental conditioning conditioning chamberchamber according to to ASTM ASTM D D5229 5229 [34] [.34 During]. During the initial the initial stage, stage,the the specimensspecimens were were subjected subjected to to drying drying at 70 ± 11 °CC for for a aperiod period of 10 of day 10 dayss in a inlaboratory a laboratory drying drying oven oven ± ◦ (SLW(SLW 53ECO, 53ECO Pol-Eko,, Pol-Eko, Wodzisław Wodzisław Sl´Śląski, ˛aski,Poland), Poland), in in order order to toremove remove moisture moisture and determine and determine the the initialinitial state state of theof the hygrothermal hygrothermal conditioningconditioning process. process. The The specimens specimens were were then then conditioned conditioned at the at the temperature of 60 °C and a 99% relative humidity to the point of reaching moisture equilibrium. temperature of 60 C and a 99% relative humidity to the point of reaching moisture equilibrium. Hygrothermal conditioning◦ tests were performed using the Binder automated environmental Hygrothermal conditioning tests were performed using the Binder automated environmental chamber chamber (MKF 115, Binder, Tuttlingen, Germany, see Figure 2). (MKF 115, Binder, Tuttlingen, Germany, see Figure2). Materials 2020, 13, x FOR PEER REVIEW 5 of 15

FigureFigure 2. The 2. The environmental environmental chamberchamber used for for hygrothermal hygrothermal conditioning conditioning..

Changes in the mass of the control samples were registered periodically at 24 h intervals (consecutively every few days) with a laboratory scale (Radwag WAS 220/X) to an accuracy of 0.0001 g. The duration of the test was 100 days. Then, samples after this exposure time were subjected to strength tests.

2.3. ILSS Test The interlaminar shear strength (ILSS) was determined by using the short beam shear test according to EN ISO14130:1997 [35]. Ten specimens were tested for each type of stacking sequence, in order to assess the effect of environmental conditions on the ILSS. The detailed test and major dimensions are presented in Figure 3.

Figure 3. Schematic configurations of the short beam shear test specimen.

The tests were performed in a Schimadzu mechanical testing machine using a test speed of 1.3 mm/min. The interlaminar shear strength (ILSS) was determined by the following Equation [35]: ퟑ푭 흉 = [MPa], (1) ퟒ풃풉 where F is the maximum load, and b and h are the width and thickness of the specimen, respectively.

2.4. Fractography The tested specimens after the short beam shear test were examined under the scanning electron microscope (SEM) for the characterization of failure modes due to the combined effect of

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Materials 2020, 13, 2999 5 of 14 Figure 2. The environmental chamber used for hygrothermal conditioning.

Changes in the mass of the control samples were registered periodically at 24 h intervals (consecutively everyevery fewfew days)days) withwith aa laboratorylaboratory scale scale (Radwag (Radwag WAS WAS 220 220/X)/X) to to an an accuracy accuracy of of 0.0001 0.0001 g. g.The The duration duration of of the the test test was was 100 100 days. days. Then, Then, samplessamples afterafter thisthis exposureexposure timetime were subjected to strength tests.

2.3. ILSS ILSS T Testest The interlaminar shear strength (ILSS) was determined by using the short beam shear test according to EN ISO14130:1997 [35] [35].. Ten Ten specimens specimens were were tested tested for for each each type type of of stacking stacking sequence sequence,, in order to assess the effect eﬀect of environmental conditions on the ILSS. The detailed test and major dimedimensionsnsions are presented inin Figure3 3..

Figure 3. SchematicSchematic configurations conﬁgurations of of the short beam shear test specimen specimen..

The tests tests were were p performederformed in in a aSchimadzu Schimadzu mechanical mechanical testing testing machine machine using using a test a test speed speed of 1.3 of mm/min.1.3 mm/min. The The interlaminar interlaminar shear shear strength strength (ILSS) (ILSS) was was determined determined by the by thefollowing following Equation Equation [35] [:35 ]: ퟑ푭 흉 = [MPa3F ], (1) ퟒ풃풉τ = [MPa], (1) 4bh where F is the maximum load, and b and h are the width and thickness of the specimen, respectively. where F is the maximum load, and b and h are the width and thickness of the specimen, respectively. 2.4. Fractography 2.4. Fractography The tested specimens after the short beam shear test were examined under the scanning electronThe microscope tested specimens (SEM) after for the shortcharacterization beam shear of test failure were examinedmodes due under to the the combined scanning effe electronct of microscope (SEM) for the characterization of failure modes due to the combined eﬀect of moisture absorption and an elevated temperature. An FEI NovaNano SEM 450 ﬁeld emission scanning electron microscope was used for SEM analysis.

3. Results and Discussion

3.1. Moisture Absorption Figure4 presents the moisture absorption for one-sided laminates (Al-C and Al-GC) and FML laminates (FML-C and FML-GC) without and with a glass composite layer subjected to exposure at 60 ◦C and 99% RH. It can be observed that moisture absorption in FML laminates is insigniﬁcant in relation to one-sided laminates (Figure4). A constant increase of moisture adsorption in the examined laminates can be observed during the entire exposure time. Moreover, it can be noticed that a typical equilibrium point does not occur in this case. This indicates that in the absorption process, anomalies and non-Fickan behavior occur. For both investigated Al-C and Al-GC laminates, the maximum moisture absorption during hygrothermal conditioning reached 0.6 mass %. In the case of FML-C and FML-GC, it can be Materials 2020, 13, 2999 6 of 14

Materialsobserved 2020 that, 13, afterx FOR c.a. PEER 100 REVIEW days, laminates absorb moisture at the level of 0.18% (see Figure4). It6 wasof 15 also noted that FML-GC absorbs slightly less moisture (0.15%) compared to the laminates with a carbon moisturefiber composite absorption (FML-C). and Aan significantly elevated temperature. lower moisture An absorptionFEI NovaNano index SEM in fiber 450 metal field laminates emission scanningin comparison electron to one-sidedmicroscope laminates was used is for caused SEM byanalysis. the outer metal layers [5,9,10]. Those aluminium layers protect FMLs from moisture absorption by decreasing the surface of the laminate open to 3.moisture. Results Forand this Discussion reason, moisture diffusion processes in fiber metal laminates only occur through free edges of the specimens (side edges). Nevertheless, due to absorption through free edges, the moisture 3.1.distribution Moisture in A thebsorption laminate may not be uniform. The literature data [36] indicates that the increase of the FMLFigure mass 4 presents at the saturation the moisture point absorption only constitutes for one- 10%sided of laminates the increase (Al for-C and traditional Al-GC) composite and FML laminatesmaterials (GFRP(FML-/CCFRP). and FML As a-GC) result, without the eff ectand of with hygrothermal a glass composite conditioning layer in subjected FMLs will to beexposure much less at 60significant °C and 99% than RH. in the case of conventional composites.

Figure 4. MoistureMoisture absorption absorption of of the the materials materials studied studied..

ItIt iscan known be observed that a disadvantage that moisture of absorption polymers, such in FML as epoxies, laminates is the is insignificant fact that they in are relation prone to oneabsorb-sided moisture laminates during (Figure exposure 4). to A humid constant environments increase becauseof moisture the epoxy adsorption matrix is in highly the examined polar [26]. laminatesMoisture absorptioncan be observed from during the atmosphere the entire exposure generally time. occurs Moreover, through it the can surface, be noticed followed that a typical by the equilibriumdiffusion process point does through not occur most ofin thethis materialcase. This [5 indicates,7]. Water that molecules in the absorption can create process hydrogen, anomalies bonds andwith non the- molecularFickan behavior structure occur. of the For matrix both during investigated absorption. Al-C Theand O-H Al-GC dipole laminates in the, waterthe maximum molecule moistureattracts hydroxyl absorption (O-H), during amine hygrothermal (N-O), sulphone conditioning (O-S), andreached phenol 0.6 (O-C)mass groups.%. In the In case the longof FML term,-C andthese FML chemical-GC, it reactions can be observed deteriorate that the after resin c.a. by 100 hydrolysis days, laminates [37]. On absorb the other moisture hand, at as the a resultlevel of 0.18%capillary (see action, Figure moisture 4). It was can also be noted transported that FML to the-GC bulk absorbs of the slightly composite less specimensmoisture (0.15%) by microcracks compared at tothe the fiber-matrix laminates interfaceswith a carbon [5,7,38 fib].e Ther composite fiber-matrix (FML interfaces-C). A significantly are responsive lower to themoisture chemical absorption effect of indexwater becausein fiber themetal glass laminates contains in alkaline comparison oxides to that one can-sided be hydrolyzed. laminates is The caused mechanical by the propertiesouter metal of layersthe glass [5,9,10] are reduced. Those byaluminium the water, layers which protect hydrates FML theses from oxides moistur [37]. e absorption by decreasing the surfaceAs of far the as laminate the fiber metalopen laminatesto moisture. are For concerned, this reason, the metalmoisture surface diffusion with an processes oxide layer in fibe alsor metal ought laminatesto be considered. only occur In the through case of metal, free edges where of the the oxide specimens layer is (side formed edges). on the Nevertheless, surface, hydration due toof absorptionthis layer may through occur free due edges, to the the moist moisture environment. distribution In turn, in the low-adhesive laminate may aluminum not be uniform. hydroxide The is literaturecreated [15 data,17,21 [36],26]. indicates Moreover, that it causes the increase a significant of the increase FML in mass volume, at the leading saturation to higher point stresses only constitutesaround the 10% part-through of the increase crack tip for and traditional subsequently composite to higher materials crack (GFRP/CFRP). growth rates [ 9As]. Ina theresult, case the of effectthe investigated of hygrothermal FML laminates, conditioning the aluminium in FMLs will surface be much was prepared less significant by anodizing than in and the applying case of conventionalprimer. Such composites. an action improves the adhesion and corrosion resistance between the composite and the aluminiumIt is known [ 10that]. Fora disadvantage this reason, dueof polymers, to stable, adherent,such as epoxies, and continuous is the fact oxide that they film [are10], prone one can to absorb moisture during exposure to humid environments because the epoxy matrix is highly polar [26]. Moisture absorption from the atmosphere generally occurs through the surface, followed by the diffusion process through most of the material [5,7]. Water molecules can create hydrogen bonds with the molecular structure of the matrix during absorption. The O-H dipole in the water molecule

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attracts hydroxyl (O-H), amine (N-O), sulphone (O-S), and phenol (O-C) groups. In the long term, these chemical reactions deteriorate the resin by hydrolysis [37]. On the other hand, as a result of capillary action, moisture can be transported to the bulk of the composite specimens by microcracks at the fiber-matrix interfaces [5,7,38]. The fiber-matrix interfaces are responsive to the chemical effect Materials 2020, 13, 2999 7 of 14 of water because the glass contains alkaline oxides that can be hydrolyzed. The mechanical properties of the glass are reduced by the water, which hydrates these oxides [37]. As far as the fiber metal laminates are concerned, the metal surface with an oxide layer also expect the anodizedought to material be considered. layers In tothe absorb case of metal, less moisture.where the oxide Anodizing layer is formed pretreatments on the surface, such as chromic hydration of this layer may occur due to the moist environment. In turn, low-adhesive aluminum or phosphoric anodizinghydroxide is created demonstrated [15,17,21,26].a Moreover, stableoxide it causes layer, a significant most increase probably in volume resulting, leading from to the presence of ions (e.g., POhigher4 for stresses the phosphoric around the part anodizing-through crack process) tip and subsequently at the surface to higher whichcrack growth prevent rates [9] oxide. hydration. Another reasonIn for the ancase improved of the investigated durability FML laminates, of the the anodized aluminium oxide surface layerswas prepared may by be anodizing the absence of certain and applying primer. Such an action improves the adhesion and corrosion resistance between the elements (e.g., magnesium)composite and the in aluminium the oxide [10]. layer For this [39 reason,]. Additionally, due to stable, adherent the primers, and continuous mostly oxide contain chromate inhibitors, whichfilm limit [10], the one canmoisture expect the di ff anodizedusion rate material through layers to the absorb primer less [ moisture.39]. Anodizing pretreatments such as chromic or phosphoric anodizing demonstrated a stable oxide layer, most In the caseprobably of conventional resulting from composites,the presence of ions and (e.g. perhaps, PO4 for the especially phosphoric anodi FMLs,zing process) where at absorption the mostly occurs throughsurface side edges, which prevent the influence oxide hydration. of reinforcing Another reason fibers for an should improved be durability considered. of the anodi Moisturezed absorption oxide layers may be the absence of certain elements (e.g., magnesium) in the oxide layer [39]. by reinforcing fibersAdditionally is inconsequential,, the primers mostly contain so they chromate work inhibitors as moisture, which limit barriers. the moisture However, diffusion applying those fibers as a reinforcementrate through the in primer composites [39]. causes important changes in the diffusion pathways of water molecules in an anisotropicIn the case of way, conventional hindering composites, simple and perhaps diffusion especially [18]. FMLs, The where varied absorption angular mostly positioning of the occurs through side edges, the influence of reinforcing fibers should be considered. Moisture fibers may influenceabsorption changes by reinforcing to the fiber characters is inconsequential, of the compositeso they work as di moistureffusion barriers. by increasing However, or decreasing the diffusion lengthapplying of those water fibers molecules as a reinforcement [18]. in Combined composites causes with important capillary change forcess in the acting diffusion in the direction pathways of water molecules in an anisotropic way, hindering simple diffusion [18]. The varied of the fibers, thisangular causes positioning a much of the higherfibers may moisture influence changes absorption to the character rate of in the the composite direction diffusion of the fibers than perpendicular toby thisincreasing direction or decreasing [9]. the diffusion length of water molecules [18]. Combined with capillary forces acting in the direction of the fibers, this causes a much higher moisture absorption rate in the direction of the fibers than perpendicular to this direction [9]. 3.2. Interlaminar Shear Strength 3.2. Interlaminar Shear Strength Figure5 shows the typical force–displacement curves of the examined laminates. The force–displacement Figure 5 shows the typical force–displacement curves of the examined laminates. The force– curves look similardisplacement for the investigated curves look similar laminate for the investi groups.gated Upon laminate reaching groups. Upon the reaching maximum the maximum load, each curve shows sharp load drops,load demonstrating, each curve shows that sharp complete load drops interlaminar, demonstrating failure that complete has taken interlaminar place. failure has taken place.

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Figure 5. Force–displacementFigure 5. Force–displacement curves of curves ﬁber of metalfiber metal laminates laminates(FMLs) (FMLs) after after interlaminar interlaminar shear shear strength strength (ILSS) tests: (a) Al-C, (b) Al-GC, (c) FML-C, and (d) FML-GC. (ILSS) tests: (a) Al-C, (b) Al-GC, (c) FML-C, and (d) FML-GC. Table 1 shows the interlaminar shear strength results for one-sided and FML laminates with Table1 showsand without the interlaminar a layer of glass shearcomposite. strength The obtained results results for show one-sided that ILSS depends and FML on both laminates the with and laminate configuration and hygrothermal conditioning. without a layer of glass composite. The obtained results show that ILSS depends on both the laminate conﬁguration and hygrothermalTable conditioning. 1. Interlaminar shear strength values for tested laminates. Before Hygrothermal Conditioning Laminate Type Al-C Al-GC FML-C FML-GC 2728 (± 125 *) 2374 (± 69) 2366 (± 84) 2326 (± 40) Fmax [N] CV ** = 4.59 CV = 2.91 CV = 3.55 CV = 1.72 ILSS [MPa] 93.0 (± 2.3) 86.4 (± 2.6) 84.6 (± 2.4) 81.5 (± 1.7) After Hygrothermal Conditioning Laminate Type Al-C Al-GC FML-C FML-GC 2299 (± 90) 1991 (± 130) 2400 (± 82) 2351 (± 75) Fmax [N] CV = 3.91 CV = 6.54 CV = 3,44 CV = 3.18 ILSS [MPa] 82.3 (± 2.7) 66.8 (± 4.1) 85.9 (± 2.0) 81.8 (± 1.4) * Standard deviation and ** sample coefficient of variation, in percent.

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Table 1. Interlaminar shear strength values for tested laminates.

Before Hygrothermal Conditioning Laminate Type Al-C Al-GC FML-C FML-GC 2728 ( 125 *) 2374 ( 69) 2366 ( 84) 2326 ( 40) Fmax [N] ± ± ± ± CV ** = 4.59 CV = 2.91 CV = 3.55 CV = 1.72 ILSS [MPa] 93.0 ( 2.3) 86.4 ( 2.6) 84.6 ( 2.4) 81.5 ( 1.7) ± ± ± ± After Hygrothermal Conditioning Laminate Type Al-C Al-GC FML-C FML-GC 2299 ( 90) 1991 ( 130) 2400 ( 82) 2351 ( 75) Fmax [N] ± ± ± ± CV = 3.91 CV = 6.54 CV = 3.44 CV = 3.18 ILSS [MPa] 82.3 ( 2.7) 66.8 ( 4.1) 85.9 ( 2.0) 81.8 ( 1.4) ± ± ± ± * Standard deviation and ** sample coeﬃcient of variation, in percent.

The data presented in Table1 shows that the interlaminar shear strength for Al-C without an additional layer of glass composite is higher than that for the Al-GC laminate containing a layer of glass laminate (93 vs. 86.4 MPa, respectively). Similarly, for FML-C and FML-GC, it can be observed that the laminates without the layer of glass composite reach slightly higher values of ILSS (84.6 vs. 81.5 MPa, respectively). Upon comparing the interlaminar shear strength in both laminates (one-sided and two-sided laminates) with and without the layer of glass composite before hygrothermal conditioning, higher ILSS values can be noted for one-sided laminates (see Table1). It can be assumed that introducing additional layers of both metal and glass composites decreases the interlaminar strength. This phenomenon may be caused by an increase of aluminium, carbon, and glass composite interfaces. Upon analysing the influence of hygrothermal conditioning, it can be observed (see Table1) that it does not cause the interlaminar shear strength to decrease for both FML-C and FML-GC laminates. In the case of one-sided laminates (Al-C and Al-GC), a significant decrease in the interlaminar shear strength after hygrothermal conditioning can be observed. For the Al-C laminate without an additional layer of glass composite, the decrease in the interlaminar shear strength after hygrothermal conditioning oscillates around 11%. For the Al-GC laminate with a layer of glass composite, a decrease of 23% was noted. Upon comparing the influence of the configuration of one-sided laminates after hygrothermal conditioning, it was noted that in the case of laminates with an additional layer of glass composite, the interlaminar shear strength decreases by 19% in comparison to laminates without the glass composite layer. According to [5,17,23,40], moisture absorption by polymer composites may cause the resistance and stiffness of laminates to decrease, which is caused by a significant plasticizing effect on the matrix with the weakening of the fiber-matrix distribution. Moreover, the water molecules enter the molecular structure, which generates residual stresses and causes the matrix to swell [37]. Therefore, as observed in this research, plasticization of the matrix and laminate distribution surfaces leads to a decrease in the interlaminar shear strength values due to hygrothermal conditioning.

3.3. Failure Analysis In order to identify the type of damage in the investigated laminates before and after hygrothermal conditioning, a microstructure analysis was conducted. Figures6–9 show characteristic forms of damage to the laminate microstructure. In the case of one-sided laminates without an additional layer of glass composite (Al-C), it was observed that the characteristic form of damage occurring before hydrothermal conditioning is only delamination in the carbon composite layer (Figure6a). Alternatively, after hygrothermal conditioning, delaminations in both the layer of carbon composite and at the metal-composite interface are observed (Figure6a,b). Materials 2020, 13, x FOR PEER REVIEW 9 of 15 Materials 2020, 13, x FOR PEER REVIEW 9 of 15 with an additional layer of glass composite, the interlaminar shear strength decreases by 19% in with an additional layer of glass composite, the interlaminar shear strength decreases by 19% in comparison to laminates without the glass composite layer. According to [5,17,23,40], moisture comparison to laminates without the glass composite layer. According to [5,17,23,40], moisture absorption by polymer composites may cause the resistance and stiffness of laminates to decrease, absorption by polymer composites may cause the resistance and stiffness of laminates to decrease, which is caused by a significant plasticizing effect on the matrix with the weakening of the which is caused by a significant plasticizing effect on the matrix with the weakening of the fiber-matrix distribution. Moreover, the water molecules enter the molecular structure, which fiber-matrix distribution. Moreover, the water molecules enter the molecular structure, which generates residual stresses and causes the matrix to swell [37]. Therefore, as observed in this generates residual stresses and causes the matrix to swell [37]. Therefore, as observed in this research, plasticization of the matrix and laminate distribution surfaces leads to a decrease in the research, plasticization of the matrix and laminate distribution surfaces leads to a decrease in the interlaminar shear strength values due to hygrothermal conditioning. interlaminar shear strength values due to hygrothermal conditioning. 3.3. Failure Analysis 3.3. Failure Analysis In order to identify the type of damage in the investigated laminates before and after In order to identify the type of damage in the investigated laminates before and after hygrothermal conditioning, a microstructure analysis was conducted. Figures 6–9 show hygrothermal conditioning, a microstructure analysis was conducted. Figures 6–9 show characteristic forms of damage to the laminate microstructure. In the case of one-sided laminates characteristic forms of damage to the laminate microstructure. In the case of one-sided laminates without an additional layer of glass composite (Al-C), it was observed that the characteristic form of without an additional layer of glass composite (Al-C), it was observed that the characteristic form of damage occurring before hydrothermal conditioning is only delamination in the carbon composite damage occurring before hydrothermal conditioning is only delamination in the carbon composite layer (Figure 6a). Alternatively, after hygrothermal conditioning, delaminations in both the layer of Materials 2020layer, 13(Figure, 2999 6a). Alternatively, after hygrothermal conditioning, delaminations in both the layer of 9 of 14 carbon composite and at the metal-composite interface are observed (Figure 6a,b). carbon composite and at the metal-composite interface are observed (Figure 6a,b).

(a) (b) (a) (b) Figure 6. Types of damage in one-sided laminates (Al-C) before (a) and after (b) hygrothermal Figure 6. Types of damage in one-sided laminates (Al-C) before (a) and after (b) hygrothermal Figure 6.conditioningTypes of damage. in one-sided laminates (Al-C) before (a) and after (b) hygrothermal conditioning. conditioning.

(a) (b) (a) (b) Figure 7. Types of damage in one-sided laminates with an additional layer of glass composite Figure 7.FigureTypes 7. ofTypes damage of damage in one-sided in one-sided laminates laminates with with an additional an additional layer layer of ofglass glass composite composite(Al-GC) (Al-GC) before (a) and after (b) hygrothermal conditioning. beforeMaterials (a(Al) and-GC) 2020 after before, 13, x ( bFOR ()a hygrothermal) andPEER after REVIEW (b) hygrothermal conditioning. conditioning. 10 of 15

(a) (b)

(c)

Figure 8. FigureTypes 8. of Types damage of damage in FML in FML laminates laminates withoutwithout an an additional additional glass glasscomposite composite layer (FML layer-C) (FML-C) before (a), (b) and after (c) hygrothermal conditioning. before (a), (b) and after (c) hygrothermal conditioning.

(a) (b)

Materials 2020, 13, x FOR PEER REVIEW 10 of 15

(a) (b)

(c)

MaterialsFigure2020, 13 8., 2999 Types of damage in FML laminates without an additional glass composite layer (FML-C) 10 of 14 before (a), (b) and after (c) hygrothermal conditioning.

Materials 2020, 13, x FOR PEER(a) REVIEW (b) 11 of 15

FigureFigure 9. 9.Types Types of of damage damage in in FML FML laminateslaminates with an additional layer layer of of glass glass composite composite (FML (FML-GC)-GC) beforebefore (a –(ac–)c and) and after after hygrothermal hygrothermal conditioning conditioning ((d).

TheThe analyses analyses of of the the damage damage of of Al-GC Al-GC laminateslaminates with with an an additional additional layer layer of of glass glass composite composite indicateindicate that, that, similar similar to the to above the presented above presented case before case hygrothermal before hygrothermal conditioning, conditioning, the delaminations the onlydelaminations occur in the only carbon occur composite in the carbon layer composite in the area layer of thein the indenter area of (Figure the indenter7a). After (Figure hygrothermal 7a). After conditioning,hygrothermal significant conditioning, delaminations significant can delaminations be observed can between be observed composite between layers composite and the metal layers and theand cracks the metal of the and oxide the layer cracks (Figure of the 7oxidea,b). layer Moreover, (Figure delaminations 7a,b). Moreover, in thedelaminations central area in of the the central carbon area of the carbon composite layer were noted. composite layer were noted. In the case of two-sided laminates without an additional layer of glass composite (FML-C) In the case of two-sided laminates without an additional layer of glass composite (FML-C) before before and after hygrothermal conditioning, a few small delaminations at the interface of the oxide and after hygrothermal conditioning, a few small delaminations at the interface of the oxide layer and layer and composite layer, as well as delaminations in the carbon composite layer, were observed composite layer, as well as delaminations in the carbon composite layer, were observed (Figure8a,b). (Figure 8a,b). It can also be observed that significant delaminations occurred at the metal-composite It caninterface alsobe in observedthe area of that the significantindenter (Figure delaminations 8c). occurred at the metal-composite interface in the area ofUpon the indenter analysing (Figure the 8 damagec). to the FML-GC laminates with an additional layer of glass compositeUpon analysing (FML-GC) the beforedamage hygrothermal to the FML-GC conditioning laminates, with delaminations an additional at layer the metal of glass-composite composite (FML-GC)interface beforewere observed. hygrothermal Moreover, conditioning, these delaminations delaminations were observed at the metal-composite in both the upper interface and lower were observed.part of the Moreover, laminate these in the delaminations area of the indenter were observed (Figure 9a, inb). both Additionally, the upper delaminations and lower part on of the the laminatesurface inof thethe areadistribution of the indenterbetween (Figurethe glass9 a,b).and carbon Additionally, composites, delaminations as well as delaminations on the surface inof the the distributioncarbon composite between the (Figure glass 9c, andd) carbon, were composites,noted. After as hygrothermal well as delaminations conditioning, in the carbon however, composite only (Figuredelaminations9c,d), were in noted. the carbon After composite hygrothermal in the conditioning, area of the in however,denter were only observed delaminations (Figure 9c). in the carbon compositeThe in behavior the area of of laminates the indenter subjected were observed to shear (Figure is a property9c). dominated by the matrix. The interlaminarThe behavior shear ofstrength laminates is controlled subjected by an to interface shear between is a property the fiber dominatedand matrix in by the the case matrix. of a Thecomposite, interlaminar while shear in the strength case of is FML controlleds, it is controlled by an interface by the between interface the between fiber and the matrix metal inand the the case ofcomposite a composite, layer while [5,36] in. The the casefractographic of FMLs, analyses it is controlled performed by themay interface indicate betweenthat the metal the metal-composite and the compositeand fiber layer-matrix [5, 36 interface]. The fractographic is susceptible analyses to the performed chemical mayaction indicate of moisture. that the In metal-composite the case of interlaminar shear, the moisture effects on composites affect the mechanical behavior of the resin and the distribution of interlaminar stresses [5]. Stresses resulting from hygrothermal environments generally reach the maximum value at the fiber-matrix interfaces and boundaries between adjacent composite layers of different fiber orientations [29]. The interface between the reinforcing fibers and the epoxy matrix, as well as between the oxide layer and epoxy matrix, plays a significant role in the effective transfer of loads in a composite. The decrease in the interfacial connection strength due to the weakening of the interface would hinder the effective transfer of loads. Unfortunately, the fiber-matrix interfaces are sensitive to the chemical effects of water. The glass contains alkaline oxides which can be hydrolyzed, possibly leading to permanent changes in the material and hence reductions in the mechanical properties of the fibers. The epoxy resin only adheres to untreated glass fibers by hydrogen bonding, while treated glass fibers form chemical and hydrogen bonds with epoxy. At the interface, voids are created because water easily debonds hydrogen bonds [9].

Materials 2020, 13, 2999 11 of 14 and fiber-matrix interface is susceptible to the chemical action of moisture. In the case of interlaminar shear, the moisture effects on composites affect the mechanical behavior of the resin and the distribution of interlaminar stresses [5]. Stresses resulting from hygrothermal environments generally reach the maximum value at the fiber-matrix interfaces and boundaries between adjacent composite layers of different fiber orientations [29]. The interface between the reinforcing fibers and the epoxy matrix, as well as between the oxide layer and epoxy matrix, plays a significant role in the effective transfer of loads in a composite. The decrease in the interfacial connection strength due to the weakening of the interface would hinder the effective transfer of loads. Unfortunately, the fiber-matrix interfaces are sensitive to the chemical effects of water. The glass contains alkaline oxides which can be hydrolyzed, possibly leading to permanent changes in the material and hence reductions in the mechanical properties of the fibers. The epoxy resin only adheres to untreated glass fibers by hydrogen bonding, while treated glass fibers form chemical and hydrogen bonds with epoxy. At the interface, voids are created because water easily debonds hydrogen bonds [9]. Residual stresses in laminates are caused by swelling and plasticizing of the epoxy matrix as a result of moisture absorption. This may cause interlaminar shear deformation, which causes micro-crack formation in the interlayers and the initiation of micro-cracks on the interface of the fiber-matrix and at the metal-composite interface, and as a result, ILSS reduction [15,18]. In good fiber-matrix interfacial adhesion systems, the swelling effect is not significant. In the case of the metal-composite interface, the moisture preferentially diffuses towards the oxide layer because both the oxide and the water are more polar. In this respect, the resin boundary layer near the interface may be important in the diffusion behavior. It is certain that the physical and mechanical properties of this resin boundary layer will differ from those of the bulk materials. However, on this issue, there is an interesting discrepancy in the literature. It is shown that this resin boundary layer is more susceptible to hydrolysis than the bulk resin because of its lower cross-link density, whereas other authors consider the boundary layer to be a diffusion barrier because of its tighter cross-linking [39]. The authors of [41] also observed that fiber-matrix debonding mainly occurred near layer boundaries, where evidence for excessive shear deformation was found. They concluded that this is a result of a resin-rich phase near the layer boundaries. This phenomenon allows for higher shear deformation when the stress state is higher than the resin yield stress. In sum, the damage analysis carried out in this work indicates the complexity of the damage process in fiber metal laminates.

4. Summary This paper has presented aspects of the interlaminar shear strength of hybrid fiber metal laminates with a carbon composite before and after hygrothermal conditioning. In this study, the aspects of the metal-composite interface of hybrid laminates with an additional layer of glass composite were discussed. The influence of environmental factors, the interlaminar shear strength, and the failure mode were researched and analysed. On the basis of the conducted research, as well as the results of the influence of moisture and temperature, it was stated that the fiber metal laminates can be counted among the materials with lower moisture adsorption compared to conventional composite materials. This phenomenon is caused by the characteristic structure of fiber metal laminates and a limited area of intensive moisture absorption on the side surfaces of the laminate. The insertion of an additional layer of glass composite decreases the interlaminar shear strength in both one- and two-sided laminates. This can be caused by the material configuration and varied properties of the selected materials: metal, carbon fiber composite, and glass fiber composite. Hygrothermal conditioning has a detrimental effect on one-sided laminates, as opposed to the two-side type of laminates, resulting in a decrease of the interlaminar shear strength. The fractographic analysis indicates that the main forms of damage in both one- and two-sided laminates are delaminations. They were observed in the carbon composite layer and at the metal-composite interface. In the case of laminates after hygrothermal conditioning, Materials 2020, 13, 2999 12 of 14 they were also observed between the layers of glass and carbon layers. It was confirmed that each type of interface in laminates is sensitive to interfacial debonding by the shear load. Further research and the development of potential alternative strategies to protect the metal-composite interface appear necessary due to the negative impact of moisture phenomena. It is necessary to prepare a surface with an appropriate metal-composite interface. High adhesive properties are required, especially in conditions of long-term exposure to moisture. This can be achieved by creating a stable oxide layer and using additional intermediate layers, e.g., a primer with a corrosion inhibitor. Sol-gel layers are currently the preferred alternative for metal surface preparation, ensuring adequate adhesion on the metal-composite interface. These layers also have a pro-ecological aspect. Another solution may be the use of composite matrix systems with low moisture absorption, especially at elevated temperatures. An important alternative may also be the use of nanomodified composites containing nanofibers (e.g., nylon), graphene, or carbon nanotubes to increase the interlaminar shear strength.

Author Contributions: J.B.: concept, state of art, results analysis, manuscript preparation, conclusions; P.J.: sample and methodology design, environmental and mechanical tests, results analysis; M.D.: sample manufacturing, microstructural analysis, results analysis, graphics preparations; B.S.: supervision, results analysis, final correction. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by National Science Center of Poland grant number UMO-2014/15/B/ST8/03447 and the APC was funded by Department of Materials Engineering, Faculty of Mechanical Engineering, Lublin University of Technology. Conflicts of Interest: The authors declare no conflict of interest. Data Availability: The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

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